The Optical Time Domain Reflectometer (OTDR) is the standard equipment for determining fault locations and loss characteristics of optical fibers. To determine the loss characteristics or fault locations, the OTDR sends out an optical pulse and clocks the return pulse reflected from a fault or from the Rayleigh backscattered process. The delay-time of the return pulse gives the spatial location of the reflection. The key issues regarding the performances of reflectometers are detection sensitivity (often called the dynamic range), spatial resolution, and the dead-zone problem. The term "dead-zone" relates to the initial length of optical fiber that is inaccessible for measurement because of the strong Fresnel reflection at the input end of the fiber. This problem plagues all time domain techniques.
To address the sensitivity issue, coherent optical time domain technique has been deployed. This approach is described by H. Izumita et al. in Journal of Lightwave Technology, vol. 12, no. 7, pages 1230-1238, 1994. The light source is a distributed-feedback semiconductor laser which requires linewidth narrowing by using optical feedback from a one kilometer length of fiber. The optical pulse is generated by a LiNbO.sub.3 electro-optic modulator. This technique offers shot-noise-limited performance but the light source may be unstable because a long fiber length is used for feedback to achieve narrow linewidth. Moreover, the dead-zone problem is not addressed. Various frequency domain reflectometry techniques have also been implemented. Frequency domain techniques typically modulates the amplitude of the light or ramping the optical frequency of the optical carrier. The technique which involves the modulation of the amplitude is described by MacDonald in Applied Optics, vol. 20., no. 10, pages 1840-1844, 1981. This technique has not been competitive and is largely abandoned. In techniques involving the ramping of the optical frequency, distributed-feedback semiconductor lasers (DFB laser) and Nd:YAG ring lasers have been used as light sources. Both techniques suffer from the fact that the optical frequency can be varied only by temperature tuning which resulted in a nonlinear and unpredictable frequency change with time. Thus, the spatial resolution is severely degraded for long fiber lengths, thus limiting its usefulness to measurements of short fibers. The technique which uses the DFB laser is described by K. Takada in IEEE Photonics Technology Letters, Vol. 4, no. 9, pages 1069-1072, 1992, and the technique which uses Nd:YAG ring laser is described by Sorin in IEEE Photonics Technology Letters, vol. 2, no. 12, pages 902-904, 1990.
Accordingly there is a need for an optical system which overcomes the detection sensitivity, spatial resolution, and the dead-zone problems left unresolved by previous efforts as described above.